The oxidation of hydrocarbons is an important reaction in industrial organic chemistry. Thus, for example, the oxidation of cyclohexane is used commercially to produce cyclohexanol and cyclohexanone, which are important precursors in the production of nylon, whereas oxidation of alkylaromatic hydrocarbons is used to produce phenol, a precursor in the production of polycarbonates and epoxy resins.
Oxidation of hydrocarbons can be conducted using well-known oxidizing agents, such as KMnO4, CrO3 and HNO3. However, these oxidizing agents have the disadvantage that their use is accompanied by the production of unwanted coupling products which can pose disposal and pollution problems.
Preferably, therefore, oxidizing agents based on peroxides or N2O are used. The cheapest oxidizing agent, however, is molecular oxygen, either in pure form or as atmospheric oxygen. However, oxygen itself is usually unsuitable for oxidizing hydrocarbons, since the reactivity of the O2 molecule, which occurs in the energetically favorable triplet form, is not sufficient.
By using redox metal catalysts it is possible to utilize molecular oxygen for oxidizing organic compounds and hence a great number of industrial processes are based on the metal-catalyzed autooxidation of hydrocarbons. Thus, for example, the oxidation of cyclohexane with O2 to cyclohexanol and/or cyclohexanone proceeds with the use of cobalt salts. These industrial processes are based on a free-radical chain mechanism, in which the bi-radical oxygen reacts with a hydrocarbon free radical, with formation of a peroxy radical and subsequent chain propagation by abstraction of an H atom from a further hydrocarbon. In addition to metal salts, however, organic molecules can also act as free-radical initiators.
However, it is a disadvantage of these processes that the selectivity decreases very greatly with increasing conversion and therefore the processes must be operated at a very low level of conversion. Thus, for example, the oxidation of cyclohexane to cyclohexanol/cyclohexanone is carried out at a conversion of 10 to 12% so that the selectivity is 80 to 85% (“Industrielle Organische Chemie” [Industrial Organic Chemistry] 1994, 261, VCH-Verlag, D-69451 Weinheim).
An alternative to metal salt catalysts is the use of organic mediators, for example N-hydroxyphthalimide (NHPI). Thus, U.S. Pat. Nos. 6,852,893 and 6,720,462 describe methods for oxidizing hydrocarbon substrates by contacting the substrate with an oxygen-containing gas, in which the oxygen content is from 5 to 100% by volume, in the presence of a free radical initiator and a cyclic imide catalyst, typically a N-hydroxycarbodiimide catalyst, such as N-hydroxyphthalimide (NHPI). The process is conducted at a temperature between 0° C. and 500° C. and a pressure between atmospheric and 100 bar (100 and 10,000 kPa). The molar ratio of the catalyst to the hydrocarbon substrate can range from 10−6 mol % to 1 mol %, whereas the molar ratio of free-radical initiator to the catalyst can be 4:1 or less, such as 1:1 to 0.5:1. Suitable substrates that may be oxidized by this process include cumene, cyclohexylbenzene, cyclododecylbenzene and sec-butylbenzene.
U.S. Pat. No. 7,038,089 discloses a process for preparing a hydroperoxide from a hydrocarbon selected from a group consisting of primary hydrocarbons, secondary hydrocarbons and mixtures thereof corresponding to said hydroperoxide which comprises conducting the oxidation of said hydrocarbon at a temperature in the range between 130 and 160° C. with an oxygen-containing gas in a reaction mixture containing said hydrocarbon and a catalyst comprising a cyclic imide compound and an alkali metal compound. Suitable hydrocarbons are said to include C4 to C20 tertiary alkanes (e.g., iso-butane, iso-pentane, iso-hexane, and the like), C7 to C20 (alkyl) aromatic hydrocarbons with 1 to 6 aromatic rings or C9 to C20 (cycloalkyl) aromatic hydrocarbons with 1 to 6 aromatic rings (e.g., xylene, cumene, ethylbenzene, diisopropylbenzene, cyclohexylbenzene, tetrahydronaphthalene (tetraline), indane, etc.), and the like. The amount of the cyclic imide compound used may be from 0.0001 to 1%, preferably from 0.0005 to 0.5 wt %, based on the reaction mixture, whereas the amount of the alkali metal compound may be from 0.000005 to 0.01%, preferably from 0.00001 to 0.005 wt % based on the reaction mixture
However, while cyclic imides have shown activity and selectivity for the oxidation of hydrocarbons to products, such as hydroperoxides, they suffer from the problem that they can be hydrolyzed to non-catalytic species in the presence of the water and organic acids that tend to be generated as common by-products of the oxidation process. Moreover, under oxidation conditions, carbon-carbon bond scission reactions can generate alkyl radicals that can react with oxygen to terminate radical chain propagation and produce undesirable organic acids, such as acetic acid. For example, acetic acid not only facilitates hydrolysis of N-hydroxycarbodiimides, but also catalyzes the exothermic decomposition of hydroperoxide products, which can result in dangerous temperature excursions as well as producing oxidation poisons.
In our co-pending U.S. Patent Application No. 60/984,271, filed Oct. 31, 2007, the entire contents of which are incorporated herein by reference, we have described a process for oxidizing hydrocarbons in the presence of a cyclic imide catalyst, in which the problem of catalyst decomposition is alleviated by controlling the reaction conditions so as to maintain the concentration of water and organic acids in the reaction medium below 50 ppm. This is achieved by operating the process at low pressure and low oxygen concentration and by passing a stripping gas through the reaction medium to strip water and organic acids from the reaction products as they are formed.
In accordance with the present invention, an alternative method of alleviating the problem of catalyst decomposition is proposed in which a slip stream of the contents of the oxidation reaction medium is removed from the oxidation reactor and is then stripped of water and organic acids before being returned to the reactor. Typically, this is achieved by reducing the pressure of the reaction medium portion to flash off the water and organic acid impurities. In this way, the overall concentration of water and organic acids can be controlled even when the oxidation reaction is run at relatively high pressures so as to increase the oxidation rate.